GBT Reveals Satellite of Milky Way in Retrograde Orbit

New observations with National Science Foundation's
Robert C. Byrd Green Bank Telescope (GBT) suggest
that what was once believed to be an intergalactic
cloud of unknown distance and significance,
is actually a previously unrecognized satellite
galaxy of the Milky Way orbiting backward around
the Galactic center.

Artist's rendition of the path of satellite galaxy Complex H (in red) in relation to
the orbit of the Sun (in yellow) about the center of the Milky Way Galaxy. The
outer layers of Complex H are being stripped away by its interaction with
the Milky Way. The hydrogen atmosphere (in blue) is shown surrounding the visible
portion (in white) of the Galaxy.CREDIT: Lockman, Smiley, Saxton;
NRAO/AUI

Jay Lockman of the National Radio Astronomy Observatory
(NRAO) in Green Bank, West Virginia, discovered that this
object, known as "Complex H," is crashing through the
outermost parts of the Milky Way from an inclined, retrograde
orbit. Lockman's findings will be published in the July 1
issue of the Astrophysical Journal, Letters.

"Many astronomers assumed that Complex H was probably
a distant neighbor of the Milky Way with some unusual
velocity that defied explanation," said Lockman. "Since its
motion appeared completely unrelated to Galactic rotation,
astronomers simply lumped it in with other high velocity
clouds that had strange and unpredictable trajectories."

High velocity clouds are essentially what their name implies,
fast-moving clouds of predominately neutral atomic hydrogen.
They are often found at great distances from the disk of the
Milky Way, and may be left over material from the formation
of our Galaxy and other galaxies in our Local Group. Over
time, these objects can become incorporated into larger
galaxies, just as small asteroids left over from the
formation of the solar system sometimes collide with the Earth.

Earlier studies of Complex H were hindered because the
cloud currently is passing almost exactly behind the outer disk
of the Galaxy. The intervening dust and gas that reside within
the sweeping spiral arms of the Milky Way block any visible light
from this object from reaching the Earth. Radio waves, however,
which have a much longer wavelength than visible light, are able
to pass through the intervening dust and gas.

The extreme sensitivity of the recently commissioned GBT allowed
Lockman to clearly map the structure of Complex H, revealing a
dense core moving on an orbit at a 45-degree angle to the plane
of the Milky Way. Additionally, the scientist detected a more
diffuse region surrounding the central core. This comparatively
rarefied region looks like a tail that is trailing behind the
central mass, and is being decelerated by its interaction with
the Milky Way.

"The GBT was able to show that this object had a diffuse 'tail'
trailing behind, with properties quite different from its main
body," said Lockman. "The new data are consistent with a model
in which this object is a satellite of the Milky Way in an
inclined, retrograde orbit, whose outermost layers are currently
being stripped away in its encounter with the Galaxy."

These results place Complex H in a small club of Galactic
satellites whose orbits do not follow the rotation of the rest
of the Milky Way. Among the most prominent of these objects are
the Magellanic Clouds, which also are being affected by their
interaction with the Milky Way, and are shedding their gas
in a long stream.

Since large galaxies, like the Milky Way, form by
devouring smaller galaxies, clusters of stars, and massive clouds
of hydrogen, it is not unusual for objects to be pulled into
orbit around the Galaxy from directions other than that of
Galactic rotation.

"Astronomers have seen evidence that this accreting material
can come in from wild orbits," said Butler Burton, an astronomer
with the NRAO in Charlottesville, Virginia. "The Magellanic
clouds are being torn apart from their interaction with the
Milky Way, and there are globular clusters rotating the wrong way.
There is evidence that stuff was going every-which-way at the
beginning of the Galaxy, and Complex H is probably left over
from that chaotic period."

The new observations place Complex H at approximately 108,000
light-years from the Galactic center, and indicate that it is
nearly 33,000 light-years across, containing approximately
6 million solar masses of hydrogen.

Radio telescopes, like the GBT, are able to observe these cold,
dark clouds of hydrogen because of the natural electromagnetic
radiation emitted by neutral atomic hydrogen at radio
wavelengths (21 centimeters).

Globular clusters, and certain other objects in the extended
Galactic halo, can be studied with optical telescopes because
the material in them has collapsed to form hot, bright stars.

The GBT is the world's largest fully steerable radio telescope.
It was commissioned in August of 2000, and continues to be outfitted
with the sensitive receivers and components that will allow it
to make observations at much higher frequencies.